1. Field of the Invention
The present invention relates to a printing head driving system for driving a print head for a serial printer and, more specifically, to a print head driving system for driving a print head having magnetic circuits which are driven to actuate corresponding print wires for printing.
2. Description of the Prior Art
A conventional print head driving system of such a kind will be described hereinafter with reference to FIGS. 4(A) to 4(C), FIGS. 5(A) to 5(C), FIG. 6 and FIG. 7.
FIGS. 4(A) to 4(C) illustrate the relationship between the arrangement of print wires in the front surface of a printing head, and dot matrices printed with the print wires.
In printing dot patterns with a single column of a plurality of print wires 1, for example, eight print wires as shown in FIG. 4(A), through an ink ribbon (not shown) on a recording sheet (not shown), it is normal to print one character in a full matrix having eight dots 2 in a line (a line along the direction of movement of the print head) as shown in FIG. 4(B) or in a pseude matrix having twelve dots 2 in a line as shown in FIG. 4(C).
The constitution of such a print head and the action of the print wires 1 will be described briefly. In the print head, each print wire 1 is attached to an armature supported on a spring plate; a core wound with a driving coil is disposed opposite to the armature with a gap therebetween; the armatures, the driving coils and the cores are arranged in a circular or elliptic arrangement, and a common permanent magnet is provided around the circular or elliptic arrangement of the cores.
Normally, the armature is attracted to the core by the magnetism of the permanent magnet to warp the spring plate. A driving current is supplied selectively to the driving coil to energize the driving coil selectively. The core associated with the energized driving coil is magnetized in a polarity to cancel the magnetism of the permanent magnet, so that the spring plate is allowed so as to release itself from the core by its own resilience and the print wire 1 thereby makes flight for printing action to print a dot.
The print speed of a serial printer provided with such a print head is dependent on the response frequency of the print wires of the print head. Therefore, the print speed of the serial printer can be enhanced by increasing the minimum dot pitch between dots printed with the same print wire during the lateral movement of the print head.
Another printing system using two columns of print wires as shown in FIG. 5(A) has been employed to enhance the print speed. As shown in FIG. 5(A), fourteen print wires 1 are arranged in the front surface of a print head in two columns spaced at a distance In printing a character "A" as shown in FIG. 4(C) while the print head is moved laterally, the dots 2 of columns 1, 2, 5, 6, and 9 are printed with the print wires 1 of the right column, while the dots 2 of columns 3, 4, 7, and 8 are printed with the print wires 1 of the left column. Columns 10 to 12 correspond to a clearance between adjacent characters, namely, a non-print area. As is obvious from FIGS. 5(B) and 5(C) showing the respective positions of the dots 2 printed with the print wires 1 in the above-mentioned printing manner, the distance d between the laterally successive dots 2 is twice the distance D between the laterally successive dots 2 printed with a single column of print wires 1 as shown in FIG. 4(C). Accordingly, the print speed of a print head provided with two columns of print wires 1 can be twice that of a print head provided with a single column of print wires 1.
In a serial printer equipped with a print head provided with two columns of print wires 1, the print head is driven at moments determined by predetermined timing on the basis of a dot pulse detected once every step of movement of a carrier mounted with the print head by a distance corresponding to D/2.
FIG. 6 is a time chart showing print head drive timing, waveforms of driving currents and the flight of print wires 1. In FIG. 6, drive timings I and II are a preceding drive timing and a succeeding drive timing, respectively, of actuating print wires 1, driving currents I and II are currents supplied to the driving coils corresponding to the print wires 1 to actuate the print wires 1 at moments determined by the drive timings I and II, respectively, and wire displacements I and II are the displacements of the print wires 1 when the driving currents are supplied to the corresponding driving coils at moments determined by the drive timings I and II, respectively.
Such a conventional print head driving system is capable of driving a print head for printing operation at a high print speed and in high print quality when the print head is comparatively large or the number of the print wires mounted on the print head is comparatively small. However, when the print head is comparatively small and has a comparatively large number of print wires, the print speed is usually reduced and the print quality is deteriorated due to magnetic interference between the magnetic circuits each including the armature, the driving coil and the core. Causes of such problems can be explained with reference to FIGS. 6 and 7.
Referring to FIG. 6, when the driving current I is supplied to a driving coil of a magnetic circuit included in a group of magnetic circuits driven by the preceding drive timing I to actuate the corresponding print wire 1 for the printing displacement I, and the driving current II is supplied subsequently to a driving coil of a magnetic circuit included in a group of magnetic circuits driven by the succeeding drive timing II to actuate the corresponding print wire 1 for the printing displacement II, the flight time, namely, time from the start of action of the print wire 1 to the arrival of the same at a print position on a recording sheet, for the displacement II increases and the starting time is delayed as indicated by broken lines because the driving time, namely, the duration of supply of the driving current I, is longer than the time interval between the driving timings I and II. Consequently, the succeeding dot 2 is dislocated from an expected position to a wrong position of a dot 2a indicated by a broken line, and hence the print speed needs to be reduced necessarily, or the print quality is deteriorated.